The most frequently employed dehydrogenation reactions involve the dehydrogenation of alkylcyclohexanes to aromatics; however, light alkane dehydrogenation is increasingly being employed. The reason for this is the growing enthusiasm for low emissions gasoline. The light alkane dehydrogenation process normally involves conversion of propane, butanes, or pentanes to the corresponding olefins, and the process configurations are similar to those utilized in catalytic reforming. As compared to catalytic reforming, the light alkane dehydrogenation processes typically operate at higher temperatures and lower pressures and with more frequent catalyst regeneration.
One of the best known methods for light alkane dehydrogenation is the so-called oxidative dehydrogenation process. In this process the light alkanes are reacted with oxygen over a suitably prepared mixed metal oxide catalyst to produce a mixture of olefin, water, CO.sub.2, and unreacted alkane. While high conversions combined with high olefin selectivities can be achieved, this process has a number of disadvantages including loss of fuel value due to water and CO.sub.2 formation and process operations that are costly and difficult from the viewpoint of industrial hazards associated with exothermic combustion reactions.
A more direct and preferred approach is direct dehydrogenation over a suitable catalyst to produce olefins and molecular hydrogen. This chemistry has recently received considerable interest, although high reaction temperatures in the range of 500.degree.-650.degree. C. are required to obtain a significant equilibrium yield (e.g., 15-50 wt. %) of olefin. Moreover, under these reaction conditions, light alkanehydrogenolysis to methane and ethane is a competing, undesirable reaction. Most catalysts studied to date have not shown very high selectivities for dehydrogenation versus hydrogenolysis or have suffered from rapid catalyst deactivation necessitating frequent regeneration. As a consequence, the process economics have not been clearly favorable. Large incentives exist for catalysts which show improved resistance to deactivation and that may be regenerated using simple procedures such as air treatment.
Prior art catalysts for direct dehydrogenation of light paraffins are mostly based on platinum on support materials such as silica, alumina, modified aluminas, and zeolites. Frequently, alkali and/or alkali earth oxide additives are included to improve stability and/or selectivity for olefin production relative to methane and ethane. One family of prior art dehydrogenation catalysts contain platinum and tin dispersed on an alumina support modified to contain alkali and/or alkali earth metals. U.S. Pat. No. 4,430,517, for example, discloses light paraffin dehydrogenation catalysts comprising a platinum group component, a Group IVA component, especially tin, an alkali or alkaline earth component, more than 0.2 wt. % of a halogen component, and a porous carrier material, wherein the atomic ratio of the alkali or alkaline earth component to the platinum group component is at least 10. Preferably, the catalyst comprises about 1 to 3 wt. % potassium. The classic Houdry-type catalyst described in UK Patent Application GB 2162082A employs chromium and potassium dispersed on alumina. By contrast, European Patent Application 212,850 discloses light paraffin dehydrogenation with catalysts containing a platinum group component on a silicalite support which is substantially free of alkali or alkali earth metals.
U.S. Pat. No. 4,547,618 discloses propane dehydrogenation catalysts comprising ZSM-12 zeolite modified with platinum and magnesium or manganese. Gallium has been noted as an important component in dehydrocyclodimerization catalysts for selective conversion of C.sub.3 and C.sub.4 alkanes to aromatics. U.S. Pat. No. 4,528,412 discloses a catalyst employing gallium dispersed in moderate acidity, ZSM-5-type zeolites for this purpose. PEP Review 85-3-3, "Aromatics from LPG," provided by SRI International, also discusses uses for this catalyst. Catalysts for the dehydrocyclodimerization process are also disclosed by A. H. P. Hall in European Patent No. 162,636. U.S. Pat. No. 4,350,835 discloses the use of Ga/H-ZSM-5 for ethane conversion to aromatics. Very recently, U.S. Pat. No. 4,985,384 has disclosed gallium containing zeolite-Beta as a catalyst for increasing aromatic yields during fluid catalytic cracking. Gallium has also been noted as a component in light alkane dehydrogenation catalysts. U.S. Pat. No. 4,056,576 discloses gallium oxide, gallium sulfate, and gallium ions exchanged onto the surface of hydrated silica or hydrated alumina, optionally modified with Pt, Pd, In, Cr, Tl, Ge, Sn, or Zn. Selectivity for propane dehydrogenation to propylene over Ga.sub.2 O.sub.3 /SiO.sub.2 at 610.degree. C. was only 71.3%. British Patent No. 1,499,297 discloses dehydrogenation of C.sub.10 + paraffins over catalysts containing platinum and gallium, indium, or thallium deposited on alumina together with minor amounts of lithium or potassium. Gallium loadings of 0.2 to 1.0 wt. % are suitable, loadings below 0.5 wt. % are preferred. Neither of these patents directly considers light paraffin dehydrogenation over bimetallic PtGa catalysts or the use of supports such as MgAl.sub.2 O.sub.4 spinels. U.S. Pat. No. 4,902,849 discloses dehydrogenation of C.sub.2 -C.sub.5 paraffins over catalysts comprising at least one aluminate spinel selected from the group consisting of aluminates of Group IIA metals and Group IIB metals, at least one metal selected from the group consisting of nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum, and at least one compound of a metal selected from the group consisting of germanium, tin, and lead. This patent does not consider the presence of Ga at all, nor is it drawn exclusively to magnesium alumina spinels.